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Ch.3 - Structure and Stereochemistry of Alkanes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.3 - Structure and Stereochemistry of AlkanesProblem 44b
Chapter 3, Problem 44b

Use a Newman projection about the indicated bond to draw the most stable conformer for each compound.
b. 3,3-dimethylhexane about the C3―C4 bond

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1
Identify the structure of 3,3-dimethylhexane. It is a six-carbon chain with two methyl groups attached to the third carbon. The main chain is hexane, and the methyl groups are substituents.
Focus on the C3―C4 bond. In a Newman projection, you will be looking down this bond, with C3 as the front carbon and C4 as the back carbon.
Draw the Newman projection circle. Place the front carbon (C3) at the center of the circle and the back carbon (C4) behind it. The front carbon will have three substituents radiating out from the center, and the back carbon will have three substituents radiating from the perimeter of the circle.
For the front carbon (C3), place the two methyl groups and one hydrogen. The methyl groups should be positioned to minimize steric hindrance, typically staggered relative to the substituents on the back carbon.
For the back carbon (C4), place the substituents (hydrogens and the rest of the hexane chain) in a staggered conformation relative to the front carbon's substituents. This staggered arrangement is the most stable conformer, minimizing torsional strain.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Newman Projection

A Newman projection is a way of drawing molecules to show the conformation around a single bond. It involves looking straight down the bond of interest, with the front atom represented by a dot and the back atom by a circle. This perspective helps visualize the spatial arrangement of substituents and identify the most stable conformations by minimizing steric hindrance.
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Introduction to Drawing Newman Projections

Steric Hindrance

Steric hindrance occurs when atoms or groups of atoms are positioned in such a way that they physically block each other, leading to increased energy and decreased stability. In Newman projections, steric hindrance is minimized by arranging bulky groups as far apart as possible, typically in staggered conformations, to achieve the most stable structure.
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Understanding steric effects.

Conformational Analysis

Conformational analysis involves studying the different spatial arrangements of atoms that result from rotation around single bonds. For 3,3-dimethylhexane, analyzing the C3―C4 bond requires identifying the most stable conformer by considering factors like steric hindrance and torsional strain, often leading to a staggered conformation where large groups are anti to each other.
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Understanding what a conformer is.
Related Practice
Textbook Question

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

c. cis-1-ethyl-3-methylcyclohexane

d. trans-1-ethyl-3-methylcyclohexane

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Textbook Question

Use a Newman projection about the indicated bond to draw the most stable conformer for each compound.

a. 3-methylpentane about the C2―C3 bond

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Textbook Question

There are eight different five-carbon alkyl groups.

c. In each case, label the degree of substitution (primary, secondary, or tertiary) of the head carbon atom bonded to the main chain.

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Textbook Question

a. Draw the two chair conformations of cis-1,3-dimethylcyclohexane, and label all the positions as axial or equatorial.

b. Label the higher-energy conformation and the lower-energy conformation.

c. The energy difference in these two conformations has been measured to be about 23 kJ (5.4 kcal) per mole. How much of this energy difference is due to the torsional energy of gauche relationships?

d. How much energy is due to the additional steric strain of the 1,3-diaxial interaction?

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Textbook Question

There are eight different five-carbon alkyl groups.

a. Draw them.

b. Give them systematic names.

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Textbook Question

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

a. cis-1-ethyl-2-isopropylcyclohexane

b. trans-1-ethyl-2-isopropylcyclohexane

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